Lightning Protection Systems Design - Part II

In the first part of this article we saw how a lightning forms and its principal characteristics, current and duration, also we saw the different criteria that are used to protect our installations. Today I will talk a little be more about this methods and how we can represent in a drawing the protection zones.
One thing that I forget to mention in the previous part is that the methods that I mentioned are used in installations of “Low Voltage” or under 69 kV since the Standard IEEE 998. It means that the conic and sphere method are useful when we are talking about houses, industrial installations and any kind of infrastructure that is not a high voltage substation.
Let’s focus in the rolling sphere method, where there are different radiuses according to standard IEC 62305. There are many formulas to determine the radio of the sphere but the ICE uses the following equation:


Where:
r: radius of the sphere.
I: Current in kA that will have the lightning when it first hits.
We will obtain a table where it shows for zones of levels of protections:

	L1	L2	L3	L4
Minimum Current [kA]	3	5	10	16
Probability of the current will be higher than the minimum [%]	99	97	91	84
Rolling Sphere Radio  [m]	20	30	45	60

From this we can deduce that higher the current, bigger will be the radio of the sphere and therefore more space can be establish between lightning rods, ironically the problem presents with the smaller currents around 3 kA, the radio of the sphere is only 20 m and it will require more lightning rods to protect the structure.
The probability that the current of the lightning overcome the minimum current gives us a warranty that in fact the protection zone described by the selected radio will work perfectly for currents higher than the used for the design; the radius that shall be used for those currents will be larger and the protection zone in that matter will be wider. For example the strictest (level 1) establishes that only 1% of the lightning could be lest that 3 kA, it is a quiet secure statement, as it is for probability.
I recommend that you read the “Lightning Protection Handbook” of Erico, there you will find further explication about that topic.
Lightning Protection Zone Drawing
Now let’s get to the best part… How do we represent the protection zone in a AUTOCAD drawing?
First of all I thing we shall see how it is NOT done.
1)     Indicated the resulting radio for the protection zone: for example if our protection level is 2, the sphere radio will be 30m, if we see a plan view, is shall NOT be simply be represented as circle of that radio with the lightning in the center given that this will be a error.
Note: the counties of measure are given in mm


Plan view – mistaken zone

What is inside the red circle should be the protection zone but there is the problem…. Lightning rods high haven’t being considered. In the following figure can be seen that for a lighting that is 15m tall (green post) and with spheres of 30 m of radio (in red) the protection zone is define is represented and it should be done in a different way, I mean, a circle is drawn centered in the lightning but the radio will be defined by the on which the sphere touches the grown.



Side view of the protection zone.

In this case is 25,98m, establishing the zone as is shown in the figure bellow, in green hatch.


Side view of the define protection zone.

Finally in the figure shown bellow we can see how the protection zone really is view from above.


Side and Plan View of the protection zone.

Even when apparently it is correct there is a detail with the plan view representation, it doesn’t indicate the high of the protection zone in particular, for example: if our house were closer to the border of the protection zone Ii will be out of it but seeing it from above won’t be notice. Watch the next figure.


Side and Plan View; structure outside the protection zone.

What we should do then is indicated the guaranteed high of the protection zone for the structures, ex: our house is 4,5 m tall, then we should find a new radio, this is done geometrically finding the point where the form of the sphere (in red) separates from the grown exactly 4,5m (yellow line).


Side view to determine the new protection zone radio.

We have now a new radio of 10,18m
Now the protection Is defined in the plan view for 4,5 m heights.

Side and Plan View. Structure outside the zone.

Now it is seen that the house is outside the zone and it is not protected.

With this we get to the final of this part, in the next post you will learn how to draw the zones of mutual influence between two or more lightning rods.

You can download the file DWG of the example Here.
Hope you liked it,
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Lightning Protection Systems Design - Part I

One of the most esoteric topics among electrical engineers is the Lightning Protection Systems, more specific lightning rods, as I have already mentioned lightings are a very complex natural phenomenon therefore is it difficult to establish and unified criteria, for this reason is that there exists a lot of opinions and strange myths that brings as result wrong lightning protection designs.
Before going any further I must confess that I am no expert in the subject, I have design a few Lightning rods using “the traditional criteria” that I´ve found in earlier documents, however I found necessary to develop a research about basic concepts. This is the results of it.
The lightning
Air is not a perfect isolating media, given that its dielectric resistance is around 30kV/cm, when a potential difference is reach between tow electrical conductor points a spark will occur inevitably (Family size, the one we call Lightning)
Depending of the polarization, the lightings are classified on negatives (electrons or negative charge ions) or positives (positive charged ions), according to its origin (Fig. 1) there are inside lightning (inside the cloud), intercloud (from cloud to cloud), clout – earth lightning (80% percent of the lightning produced and therefore the most important to us) and at last earth to cloud lightning.

Fig1. Types of lightning
Despite the short duration that they have (microseconds), lightnings have a huge destructive potential given that they carry current around 30 kA typically, up to 300 kA have been register, therefore the necessity of protecting installations and ourselves.
Lightning Formation
The lightning (this point forward it will be considered as and cloud to earth and negative) is produced by the union of the ion leaders (Fig. 2) the ascendant - Up Streamer and the descendent – Stepped Leader, they precisely are the ones that make a ionize row which is used by the lightning to go through (Fig. 3). The lightning produces when the ion leaders touch each other as seen in Fig. 4.

Fig. 2 Ascendant and Descendent Leaders

Fig. 3 – Ionize Row for by the ascendant and descendent Leaders

Fig 4. Lightning Formed
When a Lightning takes place it drains the negative charge of the cloud, it can occur a several times in a row, that why sometimes it looks like blinking in the sky.

Lightning Engineering Data

Parameter	Magnitude
First discharge amplitude	Avg. value: 30 kA, only 10% supers 75 kA
Subsequent discharge amplitude	Avg. value: 8 kA, value of the 10% - 30 kA.
Increase rate of the first discharge current	Avg. value: 7kA/us, value of the 10% 13 kA/us
Increase rate of the subsequent discharge current	Avg. value: 25 kA/us, value of the 10% 80 kA/us
Discharges by lightning	25% of the lightnings have 4 discharges, the biggest have least than 18
First discharge load	Avg. value: 6C, valor del 10% of 30C.

Protection against Atmospheric Discharges
Given that a lightning is a natural phenomenon and as one it is unpredictable, it is impossible to avoid its incidence on the structures or people 100% of the times, what a protection system does is attract the lightning that otherwise will strike in an undesired area.
The most costumed way to do so is by using lightning rods, the simplest system consist on a captor element of cooper or one with and equivalent resistance, connected solid to earth trough a isolated download wire.

The all idea is that the lightning rod has a location so it represents functions as an descendent leaders attraction element, setting it above any other structure according the location criteria.
Lightning Rod Location Criteria
Basically there are two methods to realize lightning systems designs, define the location and the amount of the lightning rods; there are the cone protection and the sphere method.
Cone Method
This method consists in setting a cone around the lightning rod and then assuming that the structures and area inside de cone will be protected.
NFPA 780 indicates two types of angles, for structures bellow 7,6m would be considered a 63 degrees angle (2 to 1 relation), for structures above 15 m would be considered a 45 degrees angle.
The next figure shows a zone considering a 45 degrees angle.

A 3D view, shows the cone formation

Applying transparency to the cone let us see the house and the person of reference.

This method is considered obsolete and lacking of scientific basement, several books consulted agree with this consideration and recommend using and electro-geometrical methods.
Rolling Sphere Method
This method derives from the call electro-geometric model (EGM); it predicts that considering an imaginary sphere of determinate radium, the lightning will have a bigger probability to strike the structures or objects that are inside of the sphere area or touch its surface, staying protected everything that it’s in the outside.
The fist reference for this method comes from the work done by Ralph H. Lee in 1997.

When the sphere is roll around the lightning rod it forms and inverted funnel as shows de next figure.

Seen in 3D.

Applying transparency it let us see the house and the reference person, inside the funnel formed by the protection zone.

The protection zone have a limited range, therefore a portion of the equipment or structure might been left unprotected.

The house on the right side is no longer been protected as it should, to overcome this problem it have to be installed another pole with a lightning rod on the top of it.

Now as we see in the figure above the house on the right side is protected by the new lightning rod, both zones overlap and forms a bigger protection zone.

Seen in 3D we have our new protection zone.

But there is an additional subject to consider, when the sphere roll between and around the two lightning rods, it creates a bigger zone than the addition of them as is shows in the figure below.

Resulting in something more likely to a circus tent.


Design considerations
According to the book: Lightning protection for engineers at the time of the calculation of the resulting protection zone it must be considered the following:
Cone Method:
1. It is a method based in a theoretical approximation and as there is been said, the lightning in most cases wont behave as it is predicted to.
2. The concept applied for this method does NOT implied safety for people since touch voltages may affect them.
3. The angle of protection is in relation to the structures high.
Rolling Sphere Method:
1. It is a method based in a theoretical approximation and as there is been said, the lightning in most cases wont behave as it is predicted to.
2. The concept applied for this method does NOT implied safety for people since touch voltages may affect them.
3. The radium of the sphere depends of the standard that have been considered:
· (US) NFPA 780, R=46m
· (US) Dept Energy and Deot Defense, R=33m

Technorati Tags: Lightning Protection Electricity Lightning Rod NFPA 780

· IEC 62305: Level I, R= 20 m / Level II, R=30m / Level III, R=45m / Level IV, R=60m
· BS 66551 (Brithis) R=20m
See the post in Spanish
 
Standards that apply to the subject:
· NFPA 780 - 2011
· IEEE Std. 998-1996. IEEE Guide for Direct Lightning
Stoke Shielding of Substations
· US Air ForceAFI 32-1065Grounding Systems (2003)
· US Air Force 91-201US Air Force Explosives Safety (2001)
· US Army385-64, Chapter 12Ammunition & Explosive Safety Standards – Lightning
· US NavyNAVSEA OP5Chapter 6, Lightning Warning and Protection (1999)
· US MilitaryMIL-STD188-124Grounding, Bonding and Shielding for Common Long Haul/Tactical Communications Systems (1992)
· US MilitaryMIL-HDBK 419A Grounding Bonding and Shielding for Electronic Equipment and Facilities (Volume I, Basic Theory, 1987)(Vol. I)
· US MilitaryMIL-HDBK 419A Grounding Bonding and Shielding for Electronic Equipment and Facilities (Volume II, Applications, 1987)(Vol. II)
· DOEM440.1-1 Department of Energy Electrical Storms and Lightning ProtectionDOEDOE/EH-0530Department of Energy Lightning Safety (1996)
· DDESBDDESB 6055.9Department of Defense Chapter 7, Lightning Protection (1997)FAAFAA-STD-019deLightning Protection, Grounding, Bonding and Shielding Requirements for Facilities (2002)
· FAAFAA 6950.19APractices & Procedures for Lightning Protection, Grounding, Bonding and Shielding Implementation (1996)
· NASA E0012EStandard for Facility Grounding and Lightning Protection (2001)
· National Weather Service30-4105Lightning Protection, Grounding, Bonding, Shielding and Surge Protection Requirements (2004)
· APIAPI-2003American Petroleum Institute Protection Against Ignitions Arising out of Static, Lightning, and Stray Currents (2008)
· British StandardBS6651Protection of Structures against Lightning (1992)
· Indian StandardIS2309Protection of Buildings and Allied Structures Against Lightning - Code of Practice (1989)(in English)
· Polish StandardPN86Lightning Protection of Structures(in Polish)7th
· AS/1768Australian Code, Lightning Protection (2004)
· Chinese Code GB50057-94Design Code for Lightning Protection of Structures(in English)
· Russian CodeRD 34.21.122-87Design Code for Buildings and Structures
 
In the part 2 it will be show how to draw the protection zones using AUTOCAD

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